In recent years, a sheet feeding unit that feeds sheets to be used in an image forming apparatus, such as printers and copying machines, is required to handle various types and sizes of the sheets and to reliably feed a large number of sheets in high speed with high accuracy. In order to reliably feed a single sheet from the stacked sheets of various types, there is known an air-assisted sheet feeding mechanism that is provided with a mechanism to blow air from a side face of the stacked sheets near the uppermost sheet at a position downstream to a position where a feed roller feeds the sheet.
In a conventional air-assisted sheet feeding mechanism, the air is blown to the side face of the stacked sheets while the uppermost sheet is held by a holding member provided at a rear end of the uppermost sheet, to thereby prevent the whole sheets in the stacked sheets float up. An example of such mechanism is disclosed in JP-4-023747-A. In the conventional mechanism, the air is blown into several tens of sheets from the uppermost sheet that is held by the holding member, to thereby form an air layer between the sheets to reduce a frictional force therebetween when feeding the sheets.
There is also known a feeding mechanism that is provided with a floating prevention member disposed at a position between an air outlet and a feed roller. An example of such mechanism is disclosed in JP-3592275-B2 (corresponding U.S. publication is: U.S. Pat. No. 6,729,614 B2).
When the air is blown into the side face of the stacked sheets in the conventional air-assisted sheet feeding mechanism, there occur a gap into which the air easily enters and a gap into which the air hardly enters due to a difference in the suction state between the sheets, to a difference in charging amount, and to a difference in a moisture absorption state. In the gap where the air hardly enters, the air would not enter across the sheets and the sheets swell in a state where the sheets partially being contact with each other. When a feed operation is performed in such state, the sheets between which the air is not duly blown may be improperly fed, thereby causing a multi feed. At worst, the sheets could not pass through separation rollers, which causes a misfeed.
As describe above, in the conventional air-assisted sheet feeding mechanism, a misfeed, such as a multi feed and an idle feed, may occur due to the suction between the sheets caused by moistures and electrostatic charge. Particularly for a case in which coated sheets having water-shedding property are used, those coated sheets absorb moistures while being exposed to a highly humid environment to be adhere to one another, whereby the multi feed and the misfeed prominently occurs.
In order to unravel the sheets adhered to one another, it is necessary to separate the adhered sheets before performing the feeding operation. There is proposed a mechanism to separate the adhered sheets by blowing air into the side face of the stacked sheets to send the air into contact surfaces between the sheets to form an air layer therebetween, to thereby reduce frictional resistance. As such mechanism, the there is known the air-assisted sheet feeding mechanism described in the above. However, each of the sheets may not be properly separated when adhesive force between the sheets differs between the sheets.